1. Field of the Invention
The invention relates to radar systems, particularly with respect to programmably controlling the range, range resolution and range gate width of an impulse radar system.
2. Description of the Prior Art
Baseband reflectometer impulse radar systems transmit a short burst of RF energy while the receiver is turned off. The receiver is then turned on at the appropriate time and for the appropriate length of time to examine a specified range for targets. Such an impulse radar system may, for example, be utilized as an intrusion detection and alert system.
Electromagnetic radiation travels at approximately one foot per nanosecond 10.sup.-9 seconds). For example, a transmitted pulse requires 100 nanoseconds to travel from the radar to a target at a distance of 100 feet / and another 100 nanoseconds to return from the tart to the radar. Thus, 200 nanoseconds are required for the round-trip pulse transit. For a receiver to detect such a target, the receiver is turned on after 200 nanoseconds have elapsed from the time the transmitter was fired, and is maintained on for a specified interval in order to receive the target return. To reduce false detections, the receiver is maintained off until the 200 nanoseconds from the time the transmitter was fired have elapsed. Similarly, for a target at a range of 150 feet, the time delay between firing the transmitter and enabling the receiver is 300 nanoseconds. Time delays appropriate for targets at different distances are determined by the one foot per nanosecond algorithm.
The interval of time that the receiver is enabled is denoted as the range gate. The width of the range gate determines the width of the range slice; i.e., the range resolution. For example, a ten foot range slice corresponds to twenty feet of round-trip path length and therefore requires a 20 nanosecond range gate. A five foot range slice provides finer range resolution and requires 10 nanoseconds of range gate time. In a system with a minimum and maximum range of interest, the narrower the range gate, the more slices exist into which to divide the range. Thus, the narrower the range gate, the finer is the resolution and the more precisely will a target be pinpointed. Prior art systems are, however, limited in producing a range gate of any particular width that will occur at controllable time intervals after firing the transmitter. Additionally, prior art systems are limited in generating sufficiently narrow range gates and hence range slices to provide high range resolution systems.
In the prior art, the time delay for each range slice is generated by counting down from preset numbers utilizing a high-speed clock. The range gate may also be generated utilizing this clock. Total range coverage is provided by a single clock in this manner since the range slices interrogated are contiguous with respect to each other. The total range is divided into equal slices, or bins, corresponding to the width of the range gate. Generally, the range gate is equivalent to one clock period and the time delay to any range bin is equal to an integral number of clock cycles.
As an illustrative example, a 100 foot range bin in a system utilizing a 20 nanosecond range gate is considered. As discussed above, a 20 nanosecond round-trip corresponds to a distance of ten feet, so that the 100 foot range bin spans the distance between 100 and 110 feet from the radar. Twenty nanoseconds is also the period of a 50 MHz clock. If the 100 foot range is divided by the width of a range bin, ten counts of the clock are required before enabling the receiver; i.e., a range delay count of 10. In this illustrative system, the count of 10 is loaded into a set of countdown counters and counting down is initiated upon firing the transmitter. When the count attains zero, the range gate is issued to enable the receiver for twenty nanoseconds in order to examine the ten foot bin. To interrogate the next range bin, the counters are loaded with the count of 11 and the sequence is repeated. In this manner, the entire range is interrogated, one range bin at a time.
The above is a simplified explanation of how prior art systems generate the range gate. If, however, the "one nanosecond per linear foot" rule were slightly inaccurate, or if there were an offset error due to a time delay between the transmit trigger and the actual transmitter firing, the only correction available in such systems would be to shift the count loaded for each range bin up or down. Such a fine correction would require an increase in the clock rate since a 50 MHz clock does not provide incremental corrections smaller than 20 nanoseconds. Additionally, if a range bin or target range resolution narrower than those described above were required, the only way to provide such increased resolution would be to increase the clock rate.
Such high clock rates are generally unattainable in practical circuitry utilizing present day technology. At 50 MHz, CMOS and most TTL is too slow. The absolute limit for Fast TTL is 100 MHz. In order to clock at these high speeds, ECL technology would be required which is expensive, necessitates careful circuit layout and utilizes excessive power compared to TTL and CMOS. The speed of ECL tops out between 125 and 150 MHz which yields a minimum range gate of 7 nanoseconds, or 3.5 feet of range resolution. To attain one foot of resolution by the above-described techniques would be prohibitive with present day digital circuitry.
The prior art also suffers from the problem that the clock speed, once selected, is fixed, and therefore the range gate and resolution are also fixed. There is no practical provision in the prior art for changing these parameters to accommodate different applications. It would be necessary to effect a hardware alteration, such as changing the clock system, in order to change these system parameters. Additionally, high speed clocks utilized in digital circuitry generate noise that has an adverse effect on the sensitivity of the radar detector.